Known casting and rolling trains convert molten steel to hot-rolled strip in a compact system. In this process, first slabs of continuous length are cast.
These slabs are cut with shears into sections, the dimensions of which correspond to a desired size of hot-rolled strip. In reheating furnaces, often designed as roller hearth furnaces, the temperature of the slabs is conditioned. The slabs are then fed separately to a rolling train, where they are rolled into strips. The strips are then cooled in a cooling zone and wound onto reels. The bundles are taken from the rolling line for further processing.
In the so-called semi-continuous process, the slabs are cut in such a way that two or more bundles can be produced from each slab. Downstream of the rolling mill, a flying shear is additionally provided for cutting the long hot-rolled strip into sections to achieve the desired bundle size. In this method, the number of critical threading-in and threading-out processes during rolling is reduced, enabling thinner hot-rolled strips to be produced more safely.
In each of these two process forms, the separation, in particular cutting, of the slabs enables the casting process and the rolling process to be carried out separately. This enables the potential and necessary processing speeds of casting machine and rolling train to be adjusted independently of one another.
Thanks to recent advances in casting machines and in process control, for example the use of heating units, it is now possible to dispense with the step of cutting the slabs into sections prior to rolling. A so-called fully continuous process has been developed. In this process, once the slab is fully solidified, it enters the rolling train without being cut into sections, while at the same time, casting on the same cast strand continues in the casting machine. The material is not separated into bundles until it reaches the flying shear downstream of the rolling train.
Thus in said fully continuous process, operating states regularly occur in which the material extending from the casting machine up to the reel is still connected as a single physical body. The entire process therefore takes place continuously or endlessly.
In mills of this size, malfunctions occasionally occur that may extend over several hundred meters. And when malfunctions occur in the hot strip rolling train, with the shears, etc., for example, the production process must be interrupted. The mill is then shut off and all movement of the strip or the slab is brought to a halt. In such cases, a strand that has not been cut into sections and has undergone different degrees of processing may extend across the entire length of the mill Since the strand is located in the various units (casting machine, shears, furnaces, rolling train, reel) over a length of 100 m or more, movement in the various regions independently of one another is impossible.
In principle, malfunctions may occur in any of the sub-units, i.e. in the areas of reeling, the flying shear(s), the finishing train, the roller hearth furnace, etc. A rolling malfunction in the finishing train caused by a strip crack between the last two stands, for example, can result extremely quickly in a material backup between these two stands that can be corrected only by manual intervention. This necessitates time-consuming work followed by inspection and in some cases repair of the mill components.
In the event of a malfunction, the mill controller or the automation system will stop the rolling process. In most cases the stands are opened up as quickly as possible, all drives are shut down and the train comes to a halt. Since the slab is still uncut up to the mold, in some cases the casting machine must also be stopped.
The casting machine as a unit is particularly critical in this context. If the stoppage continues for too long, the steel in the mold will solidify, and removing it from the mold will be highly costly and/or will result in damage to the mold. Uncontrolled opening up of the mold and the strand guide will usually result in strand breakout, causing the molten steel to pour over the unit and resulting in serious damage. And strand guiding rollers in particular are sensitive to thermal overload during prolonged idle periods.
Removing the solidified cast strand from the casting machine is highly time consuming and frequently is possible only by cutting (z. B. flame cutting) the strand manually. This requires crane work and replacement of the mold and in some cases parts of the continuous casting system. This leads to long periods of downtime and production losses, and also entails manual operations.
EP 2 259 886 B1 proposes making a cut in the strip, raising the tail of the strip that is leading in the feed direction, and cutting the subsequent strip into pieces. However this concept assumes that the subsequent strip material is still in motion. Additional or similar solutions as well as specific aspects of cutting the slab or the strip into sections are described in EP 0 625 383 B1, in DE 198 56 767 A1, in DE 42 20 424 A1, in JP 0122 4102 A, in JP 0527 7539 A, in JP 6315 7750 A and in JP 2001 276 910 A.